1. Field of the Invention
The present invention relates to an electronic flash camera having a fixed focus lens that forms an image through a scanning aperture, in general, and to such a camera wherein the size of the aperture at which the electronic flash is fired during exposure is determined by scene brightness and subject reflectivity levels, in particular.
2. Description of the Prior Art
It is well known that the sharpness of an image formed by a lens at an image plane is primarily dependent upon the exactness of focus of the image at the image plane. When a lens is focused to produce a sharp image of a particular object at the image plane, other objects that are closer or further away do not appear equally sharp. The decline in sharpness is gradual and there is a spacial zone extending in front of and behind the focused subject where the image misfocus is too small to be noticeable and therefore can be accepted as sharp. This zone is commonly referred to as the depth of field of the lens.
In the beginning, photographic cameras only employed lenses of the fixed focus type. In order to form photographic images of acceptable sharpness with such lenses over the greatest possible range of subject distances, their optical characteristics had to be chosen such that the near distance of their depth of field could only extend to within approximately 4 to 5 feet of the camera for an f/14 lens, if images of distant objects (objects at infinity) were to have an acceptably sharp focus at the camera's film plane. An image of an object produced by this type of lens that is located closer to the camera than this near distance would appear blurred or noticeably out of focus.
The production of sharp images of distant as well as relatively close objects is not a problem with an adjustable focus lens. With such a lens, the focus distance and its associated depth of field can be adjusted to produce acceptably sharp images of objects located at virtually any object distance. While an adjustable focus lens has many advantages, including the just-mentioned ability to vary its depth of field, such a lens has certain disadvantages. Among the disadvantages are cost and the increased susceptibility to mechanical failure over that of a fixed focus lens.
An electronic flash, fixed focus lens camera that is capable of forming sharp images of objects located closer to the lens than the near distance of its normal depth of field, has been disclosed. In U.S. Pat. No. 4,459,005 to Harvey, for example, exposure control apparatus for an electronic flash, fixed focus lens camera includes means for controlling exposure with either of two predetermined apertures. The apparatus includes proximity sensing means for detecting when a subject to be photographed is within a given distance of the camera and a mechanism for restricting the exposure aperture to a size that is less than the aperture size the exposure control mechanism would normally employ when the subject is within the aforementioned given distance. The proximity sensing means includes a light emitting diode for illuminating a subject to be photographed with infrared (IR) light and a sensor for measuring subject IR reflectivity. A major disadvantage inherent in this type of electronic flash exposure control apparatus is that for some subject distances, subject reflectivities, and ambient scene lighting conditions, the selected aperture size may be small enough to produce a photograph with a sharp subject image but may be too small to produce an adequately exposed subject background. For other such conditions, the selected aperture size may be large enough to produce a photograph with an adequately exposed subject background but too large to produce a subject image of acceptable sharpness.